1. Field of the Invention
This invention relates to electrostatic spraying of finely divided liquid particles and in particular it concerns novel method and apparatus for finely dividing liquid particles, applying an electrostatic charge to the divided particles and then spraying the charged particles into the atmosphere.
2. Description of the Prior Art
The patent invention is particularly suitable for the spraying of pesticides. As taught by U.S. Pat. No. 3,648,401 large areas are most effectively covered by sprayed insecticide when the major portion of the insecticide particles being sprayed are in the size range of about five to twenty microns. Particles of such small size are carried over very large distances; and, because a larger number of particles can be formed from a given amount of insecticides, when it is finely divided, the resulting spray is more likely to contact an insect than is a spray whose particles are not so finely divided. It has also been found that an effective spray of about five to twenty micron size particles is best achieved using a relatively low stream generating pressure, for example, about 0.3 kilograms per square centimeter. U.S. Pat. No. 3,900,165 describes a spraying apparatus for forming and spraying particles in the five to fifteen micron size range using a four psi air pressure. That apparatus comprises an air pressure chamber to which air is supplied at low pressure and a nozzle body located inside the chamber. The nozzle body has a series of converging spiral passages that receive air from the air chamber and direct it into a central mixing chamber which opens axially to a discharge opening from the air chamber. A liquid conduit also opens into the mixing chamber. As air enters the mixing chamber through the spiral passages it swirls in the form of a vortex and when liquid from the conduit enters the mixing chamber the swirling air breaks up the liquid into very small particles. Although the vortex velocity is high, the axial flow rate of the air, and the liquid particles carried with it, are relatively low so that the discharge velocity of the sprayed liquid is quite low.
It is also known in the prior art to apply an electrical charge to sprayed liquid particles to enhance their dispersal and to cause the particles to attract themselves to the various surfaces being sprayed e.g., insects, leaves, etc. U.S. Pat. No. 4,004,733 describes an electrostatic spray nozzle system for such liquids as agricultural pesticides and paints. Also U.S. Pat. No. 4,163,520 describes a paint spray gun wherein paint particles are sprayed in a swirling action and pass by an external wire electrode.
The prior art electrostatic spray devices have not proven satisfactory when used to spray particles smaller than about fifty microns at low spray velocity. It has been found that when very small size particles, e.g., five to twenty microns in size, are sprayed at low velocity it was not possible with prior arrangement to provide a satisfactory electrostatic charge on the particles.
The present invention overcomes the above described problem of the prior art. With the present invention it is possible to form, electrostatically charge and spray very fine particles, e.g., five to twenty microns in size, with a high degree of effectiveness.
According to the present invention there is provided a vortex type particle forming and mixing device inside a housing wherein air is directed in spiral fashion into a particle forming and mixing chamber where it forms a vortex about a longitudinal axis and breaks liquid entering the particle forming and mixing chamber into very small size particles for axial passage through a discharge opening from the chamber. A charging electrode is provided for applying an electrostatic charge to the finely divided particles. This electrode, however, rather than being located at or outside the discharge opening or around the liquid stream, as in the prior art, is located within the particle forming and mixing chamber itself; and, more specifically, it is located downstream of the liquid supply and extends into the region of the vortex formed by the incoming air stream from the spiral passages. Because of the turbulence and high velocities present in the vortex nearly all of the finely divided liquid particles are exposed to the action of the electrode so that a very uniform and complete charging of the particles is achieved. Moreover, at the location where the particles encounter the electrode charge they are elongated due to the high shearing action of the swirling air in the chamber; and this enhances their ability to receive a charge. Thereafter, surface tension makes the particles more spherical in shape so that their tendency to lose their acquired electrical charge is minimized.
In a preferred embodiment of the invention the charging electrode takes the form of a single straight wire element which enters into the particle forming and mixing chamber transversely to the longitudinal axis of the vortex flow. The electrode extends a short distance beyond the longitudinal axis of the vortex flow. The wire electrode presents minimal interference with the swirling flow in the mixing chamber and at the same time it exposes a maximum amount of the liquid particles to its charging action. The location of the electrode is just beyond the location where the incoming liquid has been broken into discrete particles but where the particles have not yet dispersed to any substantial extent. Consequently all the particles pass very close to the electrode and receive maximum charge exposure. The air velocity in this region is also very high so that any particles which do impinge on the electrode are blown off by the rapidly swirling air, and liquid buildup on the electrode is avoided.
There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described more fully hereinafter. Those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as the basis for the designing of other arrangements for carrying out the several purposes of the invention. It is important, therefore, that this disclosure be regarded as including such equivalent arrangement as do not depart from the spirit and scope of the invention.